Timothy Gray of the Department of Energy’s Oak Ridge National Laboratory led a study that may have revealed an unexpected change in the shape of an atomic nucleus. The surprise finding could affect our understanding of what holds nuclei together, how protons and neutrons interact and how elements form.

“We used radioactive beams of excited sodium-32 nuclei to test our understanding of nuclear shapes far from stability and found an unexpected result that raises questions about how nuclear shapes evolve,” said Gray, a nuclear physicist. The results are published in Physical Review Letters.

The shapes and energies of atomic nuclei can shift over time between different configurations. Typically, nuclei live as quantum entities that have either spherical or deformed shapes. The former look like basketballs, and the latter resemble American footballs.

How shapes and energy levels relate is a major open question for the scientific community. Nuclear structure models have trouble extrapolating to regions with little experimental data.

For some exotic radioactive nuclei, the shapes predicted by traditional models are the opposite of those observed. Radioactive nuclei that were expected to be spherical in their ground states, or lowest-energy configurations, turned out to be deformed.

What can turn a quantum state on its head?

In principle, the energy of an excited deformed state can drop below that of a spherical ground state, making the spherical shape the high-energy one. Unexpectedly, this role reversal appears to be happening for some exotic nuclei when the natural ratio of neutrons to protons becomes unbalanced. Yet, the post-reversal excited spherical states have never been found. It is as though once the ground state becomes deformed, all the excited states do, too.

Many examples exist of nuclei with spherical ground states and deformed excited states. Similarly, plenty of nuclei have deformed ground states and subsequent excited states that are also deformed — sometimes with different amounts or kinds of deformation. However, nuclei with both deformed ground states and spherical excited states are much more elusive.

Using data collected in 2022 from the first experiment at the Facility for Rare Isotope Beams, or FRIB, a DOE Office of Science user facility at Michigan State University, Gray’s team discovered a long-lived excited state of radioactive sodium-32. The newly observed excited state has an unusually long lifetime of 24 microseconds — about a million times longer than a typical nuclear excited state.

Long-lived excited states are called isomers. A long lifetime indicates that something unanticipated is going on. For example, if the excited state is spherical, a difficulty in returning to a deformed ground state could account for its long life.

The study involved 66 participants from 20 universities and national laboratories. Co-principal investigators came from Lawrence Berkeley National Laboratory, Florida State University, Mississippi State University, the University of Tennessee, Knoxville, and ORNL.

The 2022 experiment that generated the data used for the 2023 result employed the FRIB Decay Station initiator, or FDSi, a modular multidetector system that is extremely sensitive to rare isotope decay signatures.